The corona poling (also, “polarization”) process has been widely used in industry as a means of polarizing ferroelectric polymer thin-film materials (e.g., poly-vinylidene difluoride, PVDF; PVDF-TrFE, PMMA, TEFLON, etc.). Compared to other processing methods (e.g., contact electrode poling), corona poling is considered superior in that it does not require deposition of an additional contact poling electrode layer on the ferroelectric polymer material. When a ferroelectric polymer film does not require a contact poling electrode layer, it will have a clean surface throughout the entire corona poling process, thus leading to a finished product free from any unwanted interfacial problems, such as charge recombination sites. A polarized PVDF film without a contact poling electrode layer on a top surface can be directly used on a flat panel display. This ease-of-use could initiate a new wave of market demand for the touch-force-sensing feature on flat panel display devices in the future.
FIG. 1 shows a present state of art corona poling process chamber (100). A high voltage (e.g., from 10 kV to 50 kV) needle (101) is placed in the upper portion of the poling process chamber (100); during the corona poling process, this needle (101) serves as the electrode to excite the corona. In a typical corona poling process, atmosphere may be used as the processing ambient. Occasionally the processing ambient may be blended with certain amounts of purified N2, humidity, etc., for different processing purposes. As FIG. 1 also shows, a conductor grid (102) is placed between the high voltage needle (101) and the substrate (103). During the corona poling process, the conductor grid (102) is charged to a high voltage, whose value is higher than that of the substrate (103) but lower than that of the high voltage needle (i.e. Voltage 1 in FIG. 1). The voltage of the conductor grid (i.e. Voltage 2) is set in this manner mainly for three purposes. First, together with the high voltage needle, they establish an electric field (i.e. Edrift field in corona) in the distance between them (i.e., Dneedle to grid). Eq. (1) gives the value of such an electric field.
                              E                      drift            ⁢                                                  ⁢            field            ⁢                                                  ⁢            in            ⁢                                                  ⁢            corona                          =                                            Voltage              1                        -                          Voltage              2                                            D                          needle              ⁢                                                          ⁢              to              ⁢                                                          ⁢              grid                                                          (        1        )            
In a corona generated by the environment depicted in FIG. 1, it is this corona drift electric field Edrift field in corona that drives the needle-generated ions (e.g., 105) toward the conductor grid (102). Second, together with the grounded polymer substrate (103), the conductor grid (102) establishes another electric field (i.e. Epoling) in the distance (i.e. Dgrid to polymer) between the conductor grid (102) and polymer substrate (103), i.e.
                              E          poling                =                              Voltage            2                                D                          grid              ⁢                                                          ⁢              to              ⁢                                                          ⁢              polymer                                                          (        2        )            
The poling electric field Epoling drives the ions (e.g., 104) through the holes in the conductor grid (e.g., 106) toward the polymer substrate (103). The voltage of the conductor grid (102) also has a third effect. That is, when the ionic species (104) reach the polymer layer (103), they will charge the top surface of the polymer layer to a voltage level that is largely comparable to the conductor grid voltage. In solid state physics, this is tantamount to changing the work function of the top surface of the polymer; the bottom surface is unchanged given that the polymer is a good insulator. The deposited electrical charges (depending on the processing ambient used, they can be either positive or negative) will then be dissipated over the top surface of the polymer layer (103). When the charges reach the edges of the polymer, they will encounter processing elements (e.g., a substrate holder, or a switch specially designed to collect such charges, or the like), through which the charges will be transferred to the ground. As a result, during the presently disclosed corona poling process, the electrical charge provided by the poling current (107) and the charge lost to the ground will reach a steady state, at which time the entire top surface of the ferroelectric polymer layer will be sustained at a specific voltage value. As can be imagined, such a steady state voltage value is strongly influenced by the voltage of the conductor grid (i.e. Voltage 2); note that the distance between the conductor grid and the polymer substrate Dgrid_to polymer is so short (i.e. in the range of mm) that it can be considered as an electrical short circuit path between the two media. When the above described steady-state condition is reached, the final voltage of the top surface of the polymer layer (103) can reasonably be assumed to be that of the conductor grid (i.e. Voltage 2). As to the bottom surface of said polymer layer, since it is electrically isolated from the top surface by the thickness of the polymer layer tpolymer, the voltage value thereon will not be affected by the conductor grid voltage, i.e. it will be zero volts.
Determining the Magnitude of in-Film Electric Field in a Ferroelectric Polymer
Assuming the dielectric constant of the polymer layer (103) is close to 1, the above stated poling current (107) will establish an in-film electric field Ein-film across the top and bottom surfaces of said polymer substrate, whose value is denoted by
                              E                      in            ⁢                          -                        ⁢            film                          =                                                            V                                                      top                    —                                    ⁢                                      surface                    —                                    ⁢                  polymer                                            -                              V                                                      bottom                    —                                    ⁢                                      surface                    —                                    ⁢                  polymer                                                                    t              polymer                                =                                                    V                                                      metal                    —                                    ⁢                  grid                                            -              0                                      t              polymer                                                          (        3        )            
where Vtop_polymer_surface is the voltage of the top surface of the ferroelectric polymer material, tpolymer is the thickness of the polymer, and Ein-film is the in-film electric field across the thickness of the polymer material.
As an example, in a typical process conducted by the present system, the voltage of the conductor grid is set around 5 kV, and the thickness of the ferroelectric polymer material is in the regime of μm. For such a thin film, it will establish an in-film electric field as high as 109 volts/meter.
We now refer to schematic FIGS. 2 and 9, in which the features that can affect a corona poling process are provided. To repeat, the present system uses an in-film electric field Ein-film to pole (i.e. modify polarity by electric field) a ferroelectric polymer film. Before entering a detailed discussion, we have to identify the direction of the in-film electric field. The method of designating such a direction will be used throughout the present disclosure. As FIG. 9 shows, the in-film electric field Ein-film has a predominant directionality along the Z axis. That is, in the polymer film being poled, there is a substantially large electric field in the Z axis, but there is very little or no electric field in the X or Y-axis of the coordinate system of FIG. 9. When the thickness parameter tpolymer is in the range of μm, even a voltage of several volts suffices to establish an in-film electric field of several million volts/meter between the top and bottom surfaces of the ferroelectric polymer. Such an in-film electric field is so high that it can easily realign the dipoles (e.g., changing their directions, etc.) of a dielectric material. It is this unique ability to create dipole realignment by means of a strong in-film electric field in a single direction that polarizes, or poles, a ferroelectric polymer film. However, to make a corona poling system workable in a mass-production environment that includes delicate microelectronic devices, (such as a touch sensing feature on a flat panel display), there are several outstanding challenges, including maintaining productivity, dealing with the piezoelectric effect, product uniformity, product longevity and the like, lying before us. We will briefly discuss some of the physical/material issues that need to be dealt with.
Phase Transformations in a Ferroelectric Polymer Thin Film as a Consequence of an Extraordinarily Large in-Film Electric Field
In its bulk form, a commodity type PVDF thin film material is un-polarized in that the PVDF material is made directly out of melt. In such an un-polarized PVDF material, it is the α phase crystallite that dominates the crystalline structure of the matrix. However, to achieve the piezo-electric effect as required by a touch sensitive flat panel display, it is primarily the β phase that is useful. Thus, upon receiving a PVDF thin film that has been spray coated on a glass sheet, a method is required to transform the PVDF film from the α phase dominated matrix to one that is rich in β phase. To achieve this goal, conventional art has developed many ways to apply a substantially large electric field on the ferroelectric polymer. However, conventional art has not developed a process with which to control the α to β phase transformation. More specifically, today all that a process engineer knows is there is an abrupt increase of the population of β phase crystallites when a poling process reaches some critical condition. Indeed, since such an effect is mostly prominent in the Z axis, as has been explained earlier; so when or how this event happens is not clear to prior art, and the final value of β phase concentration will reach a plateau at an arbitrary value after the specimen has been poled by a specific electric field at a pre-defined temperature (e.g., 70-87° C. for PVDF) for a period of time (e.g., 30 min). It is still not clearly known to the industry as to how the above stated processing parameters influence one another.
Importance of Barkhausen Noise
Previous reports have disclosed that when a β phase transformation occurs, a great deal of electrical noise emanates from a ferroelectric material. This is the so-called Barkhausen noise. Most studies of Barkhausen noise has centered on metallic materials; but the study of Barkhausen noise in polymer materials has been relatively neglected and only primitive studies have been done. In fact, the relationship between Barkhausen noise and the status of phase transformation of a ferroelectric polymer thin film is very strong, and this fact is largely attributed to the extraordinarily large in-film electric field applied across a dielectric material of only a few μm in thickness. This relationship is the fundamental reason why the presently disclosed method can determine a process ending time, final polarity of a ferroelectric polymer thin film in a robust manner.
It is to be noted that what a process engineer normally investigates to determine the status of a corona poling process is the substrate current. To do a Barkhausen noise test on a ferroelectric polymer thin film, the process engineer connects a grounding wire to the ferroelectric polymer and thereafter the Barkhausen noise can be detected by an electrometer that links to the grounding wire. Meanwhile, despite the fact that studies have revealed that Barkhausen noise has many things to do with the poling process of a ferroelectric polymer thin film, the industry has not developed any effective means to take the advantage of Barkhausen noise, especially with a view towards controlling or improving the fundamental property of a ferroelectric polymer thin film. In the section of embodiments, the presently disclosed process will be associated with three examples, embodiments one, two, and three, to establish the fact that the crystalline structure of a ferroelectric polymer thin film can be manipulated by various corona poling process systems/means. For example, the performance of a PVDF film poled by a continuous type in-line corona poling system will be vastly different than that of the static, single chamber one of FIG. 3. The Barkhausen noise generated by the two types of in-line systems are also vastly different. In the past, the root causes of these variations were unclear to the process engineer. In fact, the complicated relationships between Barkhausen noise and the final characteristics of the ferroelectric polymer thin film has confused many process engineers. In the following paragraphs, the presently disclosed process will be used to elaborate their root causes, i.e. the fundamental reasons for causing said Barkhausen noise to occur/vary in different situations.
To assess the merits of a corona poling process by using Barkhausen noise to predict the ending point of said process, the directionality of the in-film electric field must be specified first, and the device used to measure said Barkhausen noise (e.g., a volt meter or current meter at a precision level of μV or nano-Amp) must be identified, so that the spikes of the Barkhausen noise can provide information meaningful for a process engineer to use. In the past, no prior art has achieved this capability. The end point of the conventional corona poling process for ferroelectric material was arbitrarily chosen (e.g., using a timer, etc.). The presently disclosed method is unique in the addition of an end point detecting feature to a corona poling process that is based on measureable, physical quantities.
FIG. 2 shows the relationship between the voltage of the conductor grid (102) and the electrical current produced by charges deposited on a ferroelectric polymer substrate (i.e. the poling current (107)) under three different voltage values of the high voltage needle, denoted in descending values as Voltage 1A, 1B, and 1C. As FIG. 2 shows, the magnitude of the poling current (107) may increase with the voltage of the conductor grid either linearly (e.g., curve 202) or non-linearly (e.g., curve 201); the shape of the curves largely depending on the voltage applied to the key components of the system (e.g., conductor grid voltage, Voltage 2 (102), and the voltage of the high voltage needle, Voltage 1 (101)). In further detail, as FIG. 2 shows, when the voltage, Voltage 1, of the high voltage needle (now denoted Voltage 1A) is much larger than that of nominal poling process condition (e.g., Voltage 1A>>Voltage 1B; a typical value of Voltage 1A can be as high as 50 kVolts), a non-linear behavior will result (denoted by curve 201). However, if the voltage of the high voltage needle is within nominal range (e.g., at Voltage 1B), the shape of the poling current curve can become a linear one (denoted by Curve 202). In a production environment, the process engineer would desire the profile of a poling current to be linear (i.e. 202). To avoid non-linear behavior, the voltage of the high voltage needle may have to be reduced to a lower value (i.e. Voltage 1C) substantially lower than that of a nominal poling condition (i.e. Voltage 1B) to prevent the poling process from “running away” (or any other uncontrollable behavior that is a result of non-linearity). This tactic pays a price—when the voltage of the high voltage needle (Voltage 1) is set too low, as curve (203) shows, the magnitude of said poling current (107) is decreased proportionally; this inevitably forces a corona poling process to require an extended processing time in order to polarize a ferroelectric polymer material completely. Whenever this happens (i.e. poling current too low), the productivity of the corona poling system is decreased. Faced with the above dilemma, non-linearity vs. extended processing time, the industry has been keenly looking for a new corona poling process, one that can add a high poling current to a ferroelectric polymer and monitor its status in an in-situ manner.
Microstructure of a PVDF Thin Film
FIG. 4 shows an experimental result, i.e., a poling current (400) characterizing a PVDF copolymer film being polarized by the presently disclosed corona poling process system. Here, the needle voltage is set at 20 kV and the conductor grid voltage is set at 7 kV, respectively. It is to be noted that, in accord with the fundamental property of ferroelectric material, there is a critical electric field for a PVDF polymer to transform α phase crystallites to β phase crystallites (e.g., 1.2 MV/cm when the temperature of the PVDF film is approximately 65° C.). When such a critical electric field condition is met, the above phase transformation process, from α to β crystallites, will take place, which results in re-aligning the polarity of the molecules embedded in the film. Note still further, the above stated polarity realigning process inevitably produces the movement of electrical charges (dipole distributions) within the bulk material. Thus, during the poling process of a ferroelectric polymer thin film, intermittent electrical current may flow through the bulk film, much like AC noise superimposed on a DC current. When the ferroelectric polymer thin film is connected to a grounding path, the substrate current (i.e. Isubstrate (3012) of FIG. 3) as measured by the current sensor (3011) is, therefore, a composite current that comprises the charge injected by the poling current ((107) of FIG. 1), trapped charges, mobile ions in the body of said polymer, and other species that may cause recombination with the poling charges. Hence, it is virtually an impossible challenge to understand the status of a corona poling process by diagnosing the form of the substrate current, let along using the result so derived to control said poling process in-situ.
Referring again to FIG. 4. As the spike (402) denotes, at the process elapse time of about 30 seconds (measured from the beginning of the poling process), the substrate current (400) surges to a magnitude that is 50% higher than that of the neighboring points (e.g., point 403). This spike (402) denotes some extraordinary event in the α to β phase transformation process within the PVDF copolymer film. If one observes the poling current (400), it can be seen that after passing the spike (402), the profile of the poling current (400) is no longer smooth, i.e., there are now numerous minor peaks in the poling current (400). Still further, once the spike (402) has occurred, the additional surges (e.g., point 404, 405, etc.) may take place throughout the rest of the poling process (i.e. denoted by segment 406), in a sporadic manner. This is because the magnitude of the in-film electric field has exceeded the above stated critical electric field and every so often an additional extraordinary event of the α to β phase transformation process may take place in said PVDF film. As the poling process proceeds, the amount of α phase crystallite available for phase transformation is gradually reduced; this is made evident by the gradually decreasing height of the corresponding spikes (e.g., 404 and 405, etc.). The slope of the poling current (400) also indicates the poling condition. At the beginning of the poling process, the slope of the substrate current (401) is quite steep; this actually indicates that the transportation process of the charges in the bulk film is dominated by the trapped charges, mobile ions, etc., rather than by the α to β phase transformation process. As the poling process proceeds, the magnitude of the electrical current contributed by the α to β phase transformation process becomes larger and more important. At point (402), the roles of the two mechanisms are balancing one another; that is, the magnitude of the substrate current contributed by the trapped charge transportation process is about the same as that generated by the α to β phase transformation process. In a corona poling process, once that point (402) is passed (the region denoted by 406), as the zig-zag profile of the substrate current (400) beyond point (402) indicates, an intense phase transformation process occurs in the PVD copolymer film. At the same time, as a result of the above described charge balancing effect, the slope of the segment (406) gradually becomes flat. Thus, point (402) literally denotes a coercivity of a ferroelectric polymer film. In FIGS. 7(A), (B), and (C), we use the parameter Ec of the corresponding hysteresis loop to characterize the above phenomenon (the sign of the current in FIGS. 4 and 7 is reversed, which does not affect the result). In FIG. 8, the steps (805), (806), and (807) of process flow (800) use the above stated characteristics to predict the ending point of a presently occurring corona poling process. As a result, a ferroelectric polymer film can be fabricated in a robust manner, making that ferroelectric property a final product of a quality unprecedented in the prior art.
FIG. 5 schematically depicts the substrate current (506) as well as its equivalent circuit loop (503) generated by a ferroelectric polymer thin film poled by the presently disclosed corona poling process system ((300) in FIG. 3). As has been explained in the previous paragraphs, there are now only two predominant sub-structures (i.e. β phase and amorphous PVDF) in a poled ferroelectric polymer material such as a PVDF thin film. As FIG. 5 shows, these two substructures can be characterized by two groups of charges, and correspondingly two variable capacitors (i.e. CDW and CCHARGE DIFFUSION) that are connecting to one another in parallel. Thus, the magnitude of the substrate current (5010, which corresponds to Isubstrate in FIG. 3) as measured by the current meter (507, which corresponds to 3011 in FIG. 3) is actually subjected to the variation of said two capacitance values (i.e. CDW and CCHARGE DIFFUSION). In practice, these two groups of charges (i.e. CDW and CCHARGE DIFFUSION) may play different roles. For example, when these two sub-structures coexist in a touch-sensitive film, it is the crystalline structure, i.e. the β phase of PVDF (i.e. the charges represented by CDW) that provides the piezoelectric effect desired by the user (e.g., in industry, most application engineers use a parameter d3j to designate the piezoelectric constant of a material in a direction denoted by 3). As to the amorphous sub-structure (i.e. whose trapped charges are represented by CCHARGE DIFFUSION), it is unwanted in that the amorphous structure does not produce any piezoelectric effect. Meanwhile, when the two sub-structures (e.g., PVDF with a copolymer ingredient) are deposited on a conventional touch sensing pad (e.g., a capacitance-sensing feature, etc.), the charges in the amorphous substructure can provide the area touched by finger with an alternative grounding path, which initiates the changes of the capacitance value. In this regard, the amorphous structure is necessary. In most of the situations, an optimal ferroelectric polymer film would be characterized by a specific concentration of both substructures. Conventional corona poling processes cannot tell the difference between the two sub-structures (i.e. β crystallites and amorphous structure) in that their individual roles and contributions to a substrate current have not been clearly understood. The microstructure of a ferroelectric polymer generated by the conventional corona poling process often turns out to be one that varies in accord with the practitioner's process history, so that different phase concentrations of α, β, γ and δ phases, may exist in a PVDF film made using different processing tools. When a ferroelectric thin film is used on a delicate microelectronic device (e.g., a touch force sensing pad), a prior art corona poling process faces an unprecedented challenge, in that the performance of the ferroelectric polymer thin film, the productivity of the corona poling system, and the capabilities of the process engineers who implement the process, all need to be simultaneously considered within a single intelligent corona poling system. This is the gap that the present disclosure is intended to close.